The present invention is related to a method of preparing a flexible electroconductive foam, and the electroconductive foam so prepared.
Flexible, electrically conducting foams are used as EMI (electromagnetic interference) shielding materials, electroconductive gaskets, gas sensors and electrostatic filters.
Two methods are known in the art for preparing flexible, electrically conductive foams.
In the first method, electrically conducting filler particles are mixed with the precursor of an otherwise electrically insulating polymer foam. The resulting foam is a combination of the electrically insulating polymer and the electrically conducting filler. Filler materials include particulate metals, particulate amorphous carbon, particulate graphite and particulate electroconductive polymers such as polypyrrole, polythiophene and polyaniline. The problem with this method is that the amount of filler that must be included in the foam in order to obtain adequate electrical conductivity is sufficient to degrade the structural and mechanical properties of the foam relative to the structural and mechanical properties of the electrically insulating base foam.
In the second method, an electrically conductive laser is formed on the surfaces of the pores of an otherwise electrically insulating polymer foam matrix. An electrically conductive material in particulate form is dispersed in a liquid (aqueous or organic) medium to form a dispersion. Examples of suitable electrically conductive materials include metals, amorphous carbon, graphite and electroconductive polymers. The dispersion is introduced to the pores of the matrix, for example by dipping the matrix in the dispersion or by painting the matrix with the dispersion. Excess dispersion is expelled from the matrix, for example by squeezing the matrix. Finally, the dispersion in the matrix pores is allowed to dry in ambient air. The liquid medium evaporates, leaving behind a layer of the electrically conducting particles on the surfaces of the pores of the matrix.
When a metal is used as the electrically conductive material, the second method provides an electrically conductive foam with a low resistivity, possibly as low as 0.1 ohm-cm, and satisfactory mechanical properties; but such electrically conductive foams are very expensive. Electrically conductive foams made by the second method using carbon or graphite as the electrically conductive material are less expensive, but the liquid medium must include a binder, such as vinyl resin acrylic resin or nitrocellulose, to bind the particles to the pore surfaces. One disadvantage of such electrically conductive foams is that it is difficult to optimize the binder concentration. High binder concentration gives better particulate adhesion to the pore surfaces and improved mechanical properties, at the expense of reduced electrical conductivity because of insulator bridge formation between the carbon or graphite particles. Low binder concentration gives poor particulate adhesion: the resulting foam tends to shed particles and so is unsuitable for some applications Another disadvantage of such electrically conductive foams is that a particulate concentration in excess of 30% often is needed to obtain adequate conductivity. Such a high particulate concentration degrades the mechanical strength, flexibility and permeability of the foam.
Electroconductive polymers, being polymers just like the matrix material, would be expected to be the most mechanically compatible with the foam matrix of all the electrically conducting materials. In practice, however, the electrically conducting layers formed using dispersions of electroconductive polymers in water and in polar organic solvents such as alcohols tend to be discontinuous, so that the resulting electrically conductive foams have relatively high resistivities. Dilute dispersions of electroconductive polymers in nonpolar solvents (for example, polyaniline in xylene) give more continuous electrically conductive layers; but nonpolar solvents cause the matrix to swell, thereby degrading the electrical conductivity of the resulting electrically conducting foam by reducing the porosity and permeability of the foam.
There is thus a widely recognized need for, and it would be highly advantageous to have, a more satisfactory method of impregnating an electrically insulating polymer foam matrix with an electroconductive polymer to produce an electrically conductive foam.